HOFFMANN, F.; JANUSSEN, D.; DR�SE, W.; ARP, G.; REITNER, J.*; University of G�ttingen, Germany; Research Institute and Museum Senckenberg, Frankfurt/M, Germany; University of G�ttingen, Germany; University of G�ttingen, Germany; University of G�ttingen, Germany: Histological investigation of organisms with hard skeletons. A case study of siliceous sponges.

Histological investigations of animals with hard skeletons represent a challenging problem in histotechnology. Siliceous and calcareous sponges commonly are treated with acid to remove the spicules prior to embedding and cutting. Methods for the investigation of spiculated sponge tissue are poorly known. Furthermore, fluorescence in situ hybridization (FISH), a key method to study symbiotic microbes, is not possible after acid treatment. For a broad range of siliceous sponge species, we developed and evaluated methods for embedding in paraffin, methylmetacrylat resins, LR White resin and cryomatrix. Different methods for cutting of tissue blocks as well as mounting and staining of the sections also were tested. Our aim was to enable histological investigations and FISH without prior removal of the spicules. To obtain an overview of tissue and skeleton arrangement, we recommend embedding of tissue blocks with LR White resin combined with en bloc staining techniques for large specimens with thick and numerous spicules, but paraffin embedding and subsequent staining for whole small specimens. For FISH on siliceous sponges, we recommend Histocryl embedding if the spicule content is high, but paraffin embedding if it is low. Until now, a standard method suitable for all histological attempts is not available.